The use of protective coatings on metal substrates for improved corrosion resistance and paint adhesion is common. Conventional techniques for coating such substrates include techniques that involve pretreating the metal substrate with a phosphate conversion coating and chrome-containing rinses. The use of such phosphate and/or chromate-containing compositions, however, imparts environmental and health concerns.
As a result, pretreatment compositions have been developed that are either free of chromate and/or phosphate, or that include phosphate at levels that are low enough to avoid the environmental and health concerns raised by conventional coating techniques. Such compositions are generally based on chemical mixtures that react with the substrate surface and bind to it to form a protective layer. For example, pretreatment compositions based on a Group IIIB or IVB metal compound have recently become more prevalent. Nevertheless, the corrosion resistance capability of these pretreatment compositions has generally been significantly inferior to conventional phosphate and/or chromium containing pretreatments.
It would be desirable to provide methods for treating a metal substrate that overcome at least some of the previously described drawbacks of the prior art, including the environmental drawbacks associated with the use of chromates and/or high levels of phosphates. It also would be desirable to provide methods for treating metal substrate that imparts corrosion resistance properties that are equivalent to, or even superior to, the corrosion resistance properties imparted through the use of conventional phosphate conversion coatings. It would also be desirable to provide related coated metal substrates.